/*
 * Copyright (C) 2017 Drakeet <drakeet.me@gmail.com>
 *
 * This file is part of rebase-android
 *
 * rebase-android is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * rebase-android is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * See the GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with rebase-android. If not, see <http://www.gnu.org/licenses/>.
 */

package com.drakeet.rebase.tool.guava;

import android.support.annotation.Nullable;
import java.io.Serializable;
import java.util.Set;

import static com.drakeet.rebase.tool.guava.Preconditions.checkNotNull;

/**
 * An immutable object that may contain a non-null reference to another object. Each instance of
 * this type either contains a non-null reference, or contains nothing (in which case we say that
 * the reference is "absent"); it is never said to "contain {@code
 * null}".
 *
 * <p>A non-null {@code Optional<T>} reference can be used as a replacement for a nullable {@code
 * T}
 * reference. It allows you to represent "a {@code T} that must be present" and a
 * "a {@code T} that might be absent" as two distinct types in your program, which can aid clarity.
 *
 * <p>Some uses of this class include
 *
 * <ul>
 * <li>As a method return type, as an alternative to returning {@code null} to indicate that no
 * value was available
 * <li>To distinguish between "unknown" (for example, not present in a map) and "known to have no
 * value" (present in the map, with value {@code Optional.absent()})
 * <li>To wrap nullable references for storage in a collection that does not support {@code null}
 * (though there are
 * <a href="https://github.com/google/guava/wiki/LivingWithNullHostileCollections">several other
 * approaches to this</a> that should be considered first)
 * </ul>
 *
 * <p>A common alternative to using this class is to find or create a suitable
 * <a href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the type in
 * question.
 *
 * <p>This class is not intended as a direct analogue of any existing "option" or "maybe" construct
 * from other programming environments, though it may bear some similarities.
 *
 * <p><b>Comparison to {@code java.util.Optional} (JDK 8 and higher):</b> A new {@code Optional}
 * class was added for Java 8. The two classes are extremely similar, but incompatible (they cannot
 * share a common supertype). <i>All</i> known differences are listed either here or with the
 * relevant methods below.
 *
 * <ul>
 * <li>This class is serializable; {@code java.util.Optional} is not.
 * <li>{@code java.util.Optional} has the additional methods {@code ifPresent}, {@code filter},
 * {@code flatMap}, and {@code orElseThrow}.
 * <li>{@code java.util} offers the primitive-specialized versions {@code OptionalInt}, {@code
 * OptionalLong} and {@code OptionalDouble}, the use of which is recommended; Guava does not
 * have these.
 * </ul>
 *
 * <p><b>There are no plans to deprecate this class in the foreseeable future.</b> However, we do
 * gently recommend that you prefer the new, standard Java class whenever possible.
 *
 * <p>See the Guava User Guide article on
 * <a href="https://github.com/google/guava/wiki/UsingAndAvoidingNullExplained#optional">using
 * {@code Optional}</a>.
 *
 * @param <T> the type of instance that can be contained. {@code Optional} is naturally covariant
 * on
 * this type, so it is safe to cast an {@code Optional<T>} to {@code
 * Optional<S>} for any supertype {@code S} of {@code T}.
 * @author Kurt Alfred Kluever
 * @author Kevin Bourrillion
 * @since 10.0
 */
public abstract class Optional<T> implements Serializable {
    /**
     * Returns an {@code Optional} instance with no contained reference.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
     * {@code Optional.empty}.
     */
    public static <T> Optional<T> absent() {
        return Absent.withType();
    }


    /**
     * Returns an {@code Optional} instance containing the given non-null reference. To have {@code
     * null} treated as {@link #absent}, use {@link #fromNullable} instead.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
     *
     * @throws NullPointerException if {@code reference} is null
     */
    public static <T> Optional<T> of(T reference) {
        return new Present<T>(checkNotNull(reference));
    }


    /**
     * If {@code nullableReference} is non-null, returns an {@code Optional} instance containing
     * that
     * reference; otherwise returns {@link Optional#absent}.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
     * {@code Optional.ofNullable}.
     */
    public static <T> Optional<T> fromNullable(@Nullable T nullableReference) {
        return (nullableReference == null)
               ? Optional.<T>absent()
               : new Present<T>(nullableReference);
    }


    Optional() {}


    /**
     * Returns {@code true} if this holder contains a (non-null) instance.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
     */
    public abstract boolean isPresent();

    /**
     * Returns the contained instance, which must be present. If the instance might be absent, use
     * {@link #or(Object)} or {@link #orNull} instead.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> when the value is absent, this method
     * throws {@link IllegalStateException}, whereas the Java 8 counterpart throws
     * {@link NoSuchElementException}.
     *
     * @throws IllegalStateException if the instance is absent ({@link #isPresent} returns
     * {@code false}); depending on this <i>specific</i> exception type (over the more general
     * {@link RuntimeException}) is discouraged
     */
    public abstract T get();

    /**
     * Returns the contained instance if it is present; {@code defaultValue} otherwise. If no
     * default
     * value should be required because the instance is known to be present, use {@link #get()}
     * instead. For a default value of {@code null}, use {@link #orNull}.
     *
     * <p>Note about generics: The signature {@code public T or(T defaultValue)} is overly
     * restrictive. However, the ideal signature, {@code public <S super T> S or(S)}, is not legal
     * Java. As a result, some sensible operations involving subtypes are compile errors:
     * <pre>   {@code
     *
     *   Optional<Integer> optionalInt = getSomeOptionalInt();
     *   Number value = optionalInt.or(0.5); // error
     *
     *   FluentIterable<? extends Number> numbers = getSomeNumbers();
     *   Optional<? extends Number> first = numbers.first();
     *   Number value = first.or(0.5); // error}</pre>
     *
     * <p>As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code
     * Optional<T>}. Casting either of the above example {@code Optional} instances to {@code
     * Optional<Number>} (where {@code Number} is the desired output type) solves the problem:
     * <pre>   {@code
     *
     *   Optional<Number> optionalInt = (Optional) getSomeOptionalInt();
     *   Number value = optionalInt.or(0.5); // fine
     *
     *   FluentIterable<? extends Number> numbers = getSomeNumbers();
     *   Optional<Number> first = (Optional) numbers.first();
     *   Number value = first.or(0.5); // fine}</pre>
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method is similar to Java 8's
     * {@code Optional.orElse}, but will not accept {@code null} as a {@code defaultValue}
     * ({@link #orNull} must be used instead). As a result, the value returned by this method is
     * guaranteed non-null, which is not the case for the {@code java.util} equivalent.
     */
    public abstract T or(T defaultValue);

    /**
     * Returns this {@code Optional} if it has a value present; {@code secondChoice} otherwise.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java
     * 8's
     * {@code Optional} class; write {@code thisOptional.isPresent() ? thisOptional : secondChoice}
     * instead.
     */
    public abstract Optional<T> or(Optional<? extends T> secondChoice);

    /**
     * Returns the contained instance if it is present; {@code null} otherwise. If the instance is
     * known to be present, use {@link #get()} instead.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
     * {@code Optional.orElse(null)}.
     */
    @Nullable
    public abstract T orNull();

    /**
     * Returns an immutable singleton {@link Set} whose only element is the contained instance if
     * it
     * is present; an empty immutable {@link Set} otherwise.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this method has no equivalent in Java
     * 8's
     * {@code Optional} class. However, this common usage: <pre>   {@code
     *
     *   for (Foo foo : possibleFoo.asSet()) {
     *     doSomethingWith(foo);
     *   }}</pre>
     *
     * ... can be replaced with: <pre>   {@code
     *
     *   possibleFoo.ifPresent(foo -> doSomethingWith(foo));}</pre>
     *
     * @since 11.0
     */
    public abstract Set<T> asSet();

    /**
     * Returns {@code true} if {@code object} is an {@code Optional} instance, and either the
     * contained references are {@linkplain Object#equals equal} to each other or both are absent.
     * Note that {@code Optional} instances of differing parameterized types can be equal.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
     */
    @Override
    public abstract boolean equals(@Nullable Object object);

    /**
     * Returns a hash code for this instance.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific choice of
     * hash code unspecified, unlike the Java 8 equivalent.
     */
    @Override
    public abstract int hashCode();

    /**
     * Returns a string representation for this instance.
     *
     * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific string
     * representation unspecified, unlike the Java 8 equivalent.
     */
    @Override
    public abstract String toString();


    private static final long serialVersionUID = 0;
}
